A nerve conduction study (NCS) is a diagnostic procedure whereby peripheral nerves are stimulated electrically and then bioelectrical potentials are recorded from the same nerve at a second location or from a muscle innervated by the activated nerve. A nerve conduction study often consists of early and late potentials. The former reflect direct conduction from the site of stimulation to the site of recording. Late potentials represent conduction from the site of stimulation antidromically towards the spinal cord—reflection along the way or in the spinal cord—and then conduction back down to the recording site.
The two most common types of late potentials associated with recording from a muscle innervated by the stimulated nerve are F-waves and A-waves. F-waves waves are highly variable waveforms that are caused by motor neuron back-firing and are generally recorded in all nerve conduction studies—whether pathology exists or not. A-waves, by contrast, have nearly constant latency and morphology and are generally not found in the absence of pathology. Thus, their presence is strongly suggestive of a focal or generalized neuropathy. The pathological entities most commonly associated with A-waves are polyneuropathies, particularly inflammatory neuropathies such as Guillain-Barre syndrome and lumbosacral nerve root compression.
One class of A-waves, called axon reflexes, are thought to be generated by collateral sprouting, have a simple morphology and are usually eliminated by supra-maximal stimulation. A-waves that persist with supramaximal stimulation, especially in multiple nerves, are sensitive indicators of electrophysiological abnormalities.
In the prior art, A-waves are identified using manual inspection of evoked late responses acquired during a nerve conduction study. Typically, a clinician views an ensemble of late responses in a raster format and makes a subjective determination as to whether an A-wave exists. This approach has several significant deficiencies:
(i) the subjective A-wave identification process is time-consuming and may not be performed because of time and resource limitations;
(ii) subjective A-wave identification does not support standardization of A-wave characteristics and thus may lead to wide differences in clinical results; and
(iii) subjective A-wave processing is, realistically, restricted to identification of the presence or absence of an A-wave—other A-wave features that may be of diagnostic value are unlikely to be extracted in reliable manner.